Prometheus

An Infernus can be encountered relatively early on if one spawns as a competitor (or in traffic) during Brucie's street races. These can be stolen, although killing an opponent and stealing their car will forfeit the race, lowering Brucie's approval rating.

The player will get a special colored Infernus from Bernie Crane for all Niko's help.

In GTA IV Multiplayer it is quite common, and can be seen driving around the streets of Liberty City.

Riding an NRG 900 around, especially around Star Junction, may cause an Infernus to spawn eventually.

Can rarely be seen on Union Drive near Middle Park East at night driving another supercar.

As with most cars, Infernuses will spawn more readily once the player is already driving one. Approaching the South Bohan safehouse from the southern bridge may be a good way to spawn parked Infernuses near a convenient storing place.

Driving a Ruiner will eventually spawn an Infernus in Bohan but it will take awhile for it to appear.

Driving a Turismo around Star Junction or southern Algonquin may eventually cause an Infernus to spawn. (Note Star Junction would be a better place to do this.)

MikeMyth

You can come across them. Not that often but very much in races. If you drive certain cars they appear, an easy way to find others if you want to get any, just drive around in nice cars, (usually ones which are usually acquired with cheats) but, can be found if you go to Grotti at your penthouse apartment. Grab a turismo and cruise around.

BashirTheTrainMaster

An Infernus can be encountered relatively early on if one spawns as a competitor (or in traffic) during Brucie's street races. These can be stolen, although killing an opponent and stealing their car will forfeit the race, lowering Brucie's approval rating.

The player will get a special colored Infernus from Bernie Crane for all Niko's help.

In GTA IV Multiplayer it is quite common, and can be seen driving around the streets of Liberty City.

Riding an NRG 900 around, especially around Star Junction, may cause an Infernus to spawn eventually.

Can rarely be seen on Union Drive near Middle Park East at night driving another supercar.

As with most cars, Infernuses will spawn more readily once the player is already driving one. Approaching the South Bohan safehouse from the southern bridge may be a good way to spawn parked Infernuses near a convenient storing place.

Driving a Ruiner will eventually spawn an Infernus in Bohan but it will take awhile for it to appear.

Driving a Turismo around Star Junction or southern Algonquin may eventually cause an Infernus to spawn. (Note Star Junction would be a better place to do this.)